I'd be interested to hear someone with more experience talk about this or if there's more recent research, but in school I read this paper: <https://research.cs.wisc.edu/vertical/papers/2013/hpca13-isa...> that seems to agree that x86 and ARM as instruction sets do not differ greatly in power consumption. They also found that GCC picks RISC-like instructions when compiling for x86 which meant the number of micro-ops was similar between ARM and x86, and that the x86 chips were optimized well for those RISC-like instructions and so were similarly efficient to ARM chips. They have a quote that "The microarchitecture, not the ISA, is responsible for performance differences."

`lea` is a very common x86 instruction that isn't RISC-like. (Actually I don't think any x86 operations are RISC-like since they're variable length and overwrite their inputs.)

It's just that the most complicated of all x86 instructions are so specific that they're too irrelevant to use. Or were straight up removed in x86-64.